The redshift evolution of the S0 fraction for z < 1 in COSMOS

Abstract

ABSTRACT Lenticular (S0) galaxies are galaxies that exhibit a bulge and disc component, yet lack any clear spiral features. With features considered intermediary between spirals and ellipticals, S0s have been proposed to be a transitional morphology, however their exact origin and nature is still debated. In this work, we study the redshift evolution of the S0 fraction out to z ∼ 1 using deep learning to classify F814W (i band) Hubble Space Telescope-Advanced Camera for Surveys (HST-ACS) images of 85 378 galaxies in the Cosmic Evolution Survey (COSMOS). We classify galaxies into four morphological categories: elliptical (E), S0, spiral (Sp), and irregular/miscellaneous (IrrM). Our deep learning models, initially trained to classify Sloan Digital Sky Survey (SDSS) images with known morphologies, have been successfully adapted to classify high-redshift COSMOS images via transfer learning and data augmentation, enabling us to classify S0s with superior accuracy. We find that there is an increase in the fraction of S0 galaxies with decreasing redshift, along with a corresponding reduction in the fraction of spirals. We find a bimodality in the mass distribution of our classified S0s, from which we find two separate S0s populations: high-mass S0s, which are mostly red and quiescent; and low-mass S0s, which are generally bluer and include both passive and star-forming S0s, the latter of which cannot solely be explained via the faded spiral formation pathway. We also find that the S0 fraction in high-mass galaxies begins rising at higher z than in low-mass galaxies, implying that high-mass S0s evolved earlier.